The gradient is elevated when the alveolar function is impaired and ventilation issues appear such as in pulmonary embolism or right to left shunt. A normal gradient accompanied by hypoxemia (low P aO 2) indicates hypoventilation by decreased respiratory drive or neuromuscular impairment or the possibility of a low fraction of oxygen inspired. It is used to determine whether the source of hypoxemia is intra or extra pulmonary. The normal range is between 5 to 20 mmHg up to middle age and increases as we get older in a rhythm of 1 mmHg for every decade. Medical implications of the A-a gradient: ■ Usually A-a Gradient increases 5 to 7 mmHg for every 10% increase in F iO 2. ■ Expected (normal) A-a Gradient for the specified age is 9.5 mmHg or 1.27 kPa.
Let’s take the example of a patient aged 34, P aO 2 = 86 mmHg, P aCO 2 = 45 mmHg and P atm = 760 mmHg, F iO 2 = 21%. The other two variables used are the water vapor pressure pH2O (mmHg) = 47 mm Hg at 37 degrees Celsius and the respiratory quotient RQ (VCO2/VO2) = 0.8.Ī-a gradient = – P aO2 Example calculation Fraction of Inspired Oxygen (F iO 2) – is set default at 21% as the concentration in room air. Atmospheric pressure – is set default at 760 mmHg, the value for above sea level. Age – the age of the subject is required in order to determine the estimated normal A-a gradient based on the following formula: (Age + 4) / 4 High values are suggestive for impaired ventilation while low values indicate hypocapnia or overventilation. It ranges between 35-45 mmHg and is the main indicator of CO2 production and elimination. CO 2 Arterial pressure – P aCO 2 is the partial arterial pressure of carbon dioxide in the arteries. Low levels are indicator of hypoxia, a level of 60 mmHg requires additional oxygen and anything dropping below 30mmHg is life threatening. O 2 Arterial pressure – P aO 2 is the partial arterial pressure of the oxygen in the arteries its range is between 75-100 mmHg. The following lines are a guide to how you should use this A-a gradient calculator: For your convenience, you can input the pressures required in either mmHg or kPa. This gradient is the difference between the alveolar concentration of oxygen and the arterial concentration of oxygen.
This tool assesses the alveolar - arterial gradient and determines the possible existence and the source of hypoxemia. How does this A-a gradient calculator work?
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